Method of Casting an Article

ABSTRACT

A method of casting an article using a mold including a mold cavity, the method including the steps of providing a compound containing a halogen at at least a portion of a surface of the mold adjacent the mold cavity, pouring molten metal into the mold cavity and allowing the metal to cool and solidify, and removing the article from the mold.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application hereby claims the benefit of pending InternationalApplication No. PCT/GB2005/004712, filed Dec. 8, 2005, (and publishedJun. 22, 2006, as WO 2006/064188 A1), which itself claims the benefit ofpending United Kingdom Application No. 0427501.2, filed Dec. 16, 2004,(and published Jun. 21, 2006, as GB 2,421,207 A). These applications arehereby incorporated by reference in their entirety. InternationalApplication No. PCT/GB2005/004712 designates the United States and,accordingly, this U.S. non-provisional application is a continuationthereof.

FIELD OF THE INVENTION

The present invention relates to a method of casting an article,particularly, but not exclusively to a method of sand-casting analuminum alloy engine block.

BACKGROUND OF THE INVENTION

Typically, an engine block is made by sand casting an aluminum alloy. Inorder to provide a suitable low friction, wear resistant, reciprocatingsurface, it is known to line the surface of each cylinder in the engineblock in which a piston reciprocates, with a tubular cast iron liner.Such liners increase the weight of the engine and, as cast iron has alower thermal conductivity than aluminum based alloys, the use of suchliners is detrimental to the removal of heat from the engine cylinders.

Thus, in order to remove the need for cast iron liners, variousprocesses of coating the inner surface of each cylinder with a metalthat is harder than aluminum to provide a low friction, wear resistantreciprocating surface have been developed, and these includeelectro-coating, plasma transfer wire arc coating, and laser alloying.For example, the plasma-spraying of a ferrous coating is described inU.S. Pat. No. 6,548,195, and use of laser alloying is described in U.S.Pat. No. 6,390,050 and European Publication No. EP 1041173 A1.Unfortunately, such processes only produce a satisfactory coating ifapplied to a relatively smooth substrate, and even after machining, thesurface of a sand cast engine block includes significant porosity.

This problem has been addressed in U.S. Pat. No. 5,931,213, whichdiscloses a method of casting an engine block of aluminum in whichcylindrical brass inserts are inserted into a sand mold, the brassinserts forming the cylinders in the engine block. As a result of theuse of such high thermal conductivity inserts, the cooling rate of thealuminum alloy around the inserts is significantly higher than thecooling rate of the aluminum alloy elsewhere in the mold cavity.Consequently, the porosity in the resultant casting is significantlyreduced in the volumes around the inserts, and, once the inserts areremoved, machining of the interior of each cylinder produces a surfacethat is suitable for coating using the above-mentioned methods.

This method has a number of disadvantages, however. Removal of the brassinserts can be difficult, particularly in a V-engine where the cylindersare inclined relative to the engine block, and adds an extra, relativelylabor-intensive step in the manufacturing process. Moreover, the brassinserts are susceptible to mechanical damage, particularly afterrepeated exposure to molten aluminum alloy has caused the brass toanneal and soften, and must be replaced at regular intervals, whichfurther increases the cost of the casting process.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, we provide a method ofcasting an article using a mold, including a mold cavity. The methodincludes the steps of providing a compound containing a halogen at atleast a portion of a surface of the mold adjacent the mold cavity,pouring molten metal into the mold cavity, allowing the metal to cooland solidify, and removing the article from the mold.

By virtue of the provision of such a compound, porosity formed byreaction at the surface of the casting adjacent the relevant portion ofmold is reduced, and once machined, this surface provides a suitablesubstrate for electro-plating, plasma transfer wire arc coating etc. toproduce a low friction, wear resistant surface.

Preferably, the halogen is fluorine. The coating may include, forexample, dipotassium fluorotitanate. Alternatively, the halogen may bechlorine, and may be, for example, a potassium chloride-magnesiumchloride eutectic.

Preferably, the halogen containing compound is provided in a coatingapplied to at least a portion of the mold surface which forms at leastpart of the mold cavity.

Preferably, the method further includes the step of subjecting thearticle to isostatic pressure. In this case, preferably, the methodfurther includes the step of heating the article to a temperature atwhich the entire article remains solid while applying the isostaticpressure.

Subjecting the article to an isostatic pressure, particularly at highpressure, generally eliminates internal pores within the cast article,which improves the mechanical integrity of the article and reduces therisk of any such pores being exposed at the article surface duringmachining of the article.

The metal is preferably predominantly aluminum. The metal may be analuminum-silicon alloy, and is preferably a hypoeutecticaluminum-silicon alloy.

Preferably, the coating is sprayed onto the mold surface.

Preferably, the mold is made predominantly from sand.

The article may be an engine block. In this case, preferably, the coatedportions of the mold form cylinders in the engine block.

According to a second aspect of the invention, we provide an articlecast using the method of the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of the method of making an articleaccording to the invention.

DETAILED DESCRIPTION

In accordance with an exemplary embodiment of the invention and asdepicted in FIG. 1, the method is applied to sand casting an engineblock 10 from a conventional hypoeutectic aluminum-silicon alloy, suchas the 356 or 354 alloys detailed in Tables 1 and 2 (below): TABLE 1(356 Alloy) Min. Max. Element (weight fraction) (weight fraction) IdealContent Cu — 0.2% N/A Mg 0.4% 0.6% >0.5% Si 6.5% 7.5% >7.00% Fe — 0.5%N/A Mn — 0.3% >0.2% Ni — 0.1% N/A Zn — 0.10% N/A Pb — 0.10% N/A Sn —0.05% N/A Ti  0.05% 0.15% >0.10% Sr 200 ppm 350 ppm 250 ppm P —  20 ppmN/A Na —  20 ppm N/A Ca — 100 ppm N/A Sb — 200 ppm N/A Li —  2 ppm N/AAl + additional Balance Balance Balance impurities

TABLE 2 (354 Alloy) Min. Max. Element (weight fraction) (weightfraction) Ideal Content Cu 1.5% 2.0% 1.80% Mg 0.5% 0.6% 0.55% Si 8.5%9.5% 9.00% Fe — 0.15% 0.10% Mn — 0.05% 0.00 Ni — 100 ppm 0.00 Zn — 0.05%0.00 Pb — 100 ppm 0.00 Sn — 100 ppm 0.00 Ti 0.11% 0.14% 0.12% Sr 200 ppm250 ppm 225 ppm P —  20 ppm 0.00 Na — 100 ppm 0.00 Ca — 100 ppm 0.00 Sb—  20 ppm 0.00 Li — 100 ppm 0.00 Al + additional Balance Balance Balanceimpurities

It should be appreciated, however, that the method may equally beapplied to the making of any cast article using any metal or metalalloy, such as an aluminum-magnesium alloy, or magnesium based alloys.

A mold 12 is formed from first 12 a and second 12 b mold parts made fromzircon or silicon sand using conventional sand-casting techniques. Thecylindrical portions of an upper mold part 12 a, which form cylindricalcavities, i.e., the cylinders in the engine block, are then coated witha coating 14 containing a halogen, such as fluorine. In this example,the halogen is fluorine, and the coating material contains dipotassiumfluorotitanate powder (K₂TiF₆). Other halogen containing compounds, suchas a mixed potassium chloride—magnesium chloride eutectic, potassiumborofluoride, or aluminum chloride may alternatively be used.

The K₂TiF₆ powder is mixed with a solvent, a filler material such aszircon powder, and a gelling agent. The coating material, for example,may comprise 60 weight percent powder (the powder comprising 25 weightpercent zircon flour and 75 weight percent K₂TiF₆ dry milled powder) and40 weight percent IPA solvent.

The coating is typically sprayed onto the mold surface, but may also bepainted onto the mold. Alternatively, dry K₂TiF₆ powder may be rubbedonto the mold surface, or even added directly to the sand used to formthe mold at levels of approximately 0.5 weight percent K₂TiF₆.

The mold parts 12 a and 12 b are clamped together to form a mold cavity,and molten aluminum-silicon alloy is pored into the mold cavity. Themold is then allowed to cool until the alloy has solidified. The as-castengine block 10 is then removed from the mold 12.

Usually during sand-casting of an alloy such as a hypoeutecticaluminum-silicon (Al—Si) alloy, hydrogen ions migrate from the mold 12into the liquid metal adjacent the mold surface. As the molten metalsolidifies, dissolved hydrogen is ejected from the solidification frontinto the remaining liquid metal, which results in the formation of aplurality of sub-surface elongate micropores that extend into thecasting, generally perpendicular to the surface of the casting, up to adepth of 3 to 4 mm. The inclusion of strontium in the alloy is believedto enhance this process. Conventionally, during machining of the castarticle, less than 3 mm of material is removed from the article surface,and therefore such machining exposes these pores at the article surface,and renders the surface unsuitable for coating using plasma transferwire arc coating, electro-plating and similar processes.

Where the K₂TiF₆ coating is present, however, the coating reducestransfer of hydrogen ions into the molten alloy and thereforesignificantly reduces the surface and sub-surface microporosity of thecasting. It is believed that this occurs because the fluorine in thecoating reacts with the hydrogen ions before they can dissolve in themolten alloy. As a result, the surface of the casting adjacent thecoating, in this example the interior surfaces of the cylinders in theengine block, may be machined and coated as described above withoutsurface microporosity having a deleterious effect on the integrity ofthe coating.

Even where a K₂TiF₆ coating is used, such a casting will, however,include internal pores, formed not as a result of reaction with the moldas described above, but as a result of the relatively low cooling rateassociated with sand casting. The internal pores may be exposed at thearticle surface during machining of the casting, and therefore it isdesirable to eliminate these pores in addition to the surfacemicroporosity.

The as-cast engine block is therefore subjected to hot isostaticpressing using a conventional, commercially available Al HIPPINGprocess, the Bodycote Densal® II process, for example. In such aprocess, the engine block is placed in a container of fluid, heated to atemperature close to the melting temperature of the alloy but at whichthe alloy remains solid (e.g., 40° C. below the solidus temperature ofthe alloy), and the fluid pressurized (e.g., to 1,000 atmospheres ofpressure). The engine block is typically retained in the pressurized,heated fluid for forty five minutes to one hour.

The combination of fluid pressure and elevated temperature causes theinternal pores to cave in and the material formerly surrounding eachpore to diffusion bond. Thus, the internal pores are substantiallyeliminated and the mechanical integrity of the casting improved. Thecasting may thus be machined with substantially reduced risk of exposinginternal pores at the casting surface.

It should be appreciated that HIPPING cannot be used to remedy surfacemicroporosity, since the pressurized fluid would fill the surface poresand provide internal support preventing the pore from caving in.Moreover, HIPPING is ineffective in eliminating sub-surfacemicroporosity, since the layer of aluminum alloy separating each porefrom the article surface is relatively thin, and is easily rupturedunder the pressure of the pressurized fluid, thus exposing the poreinterior to the pressurized fluid and preventing consolidation of thematerial around the pore. In order to address the problem ofsurface/sub-surface porosity and internal porosity, it is thereforenecessary to use a mold coated with or containing a halogen containingcompound in addition to HIPPING.

After HIPPING, the casting is then machined to the required dimensionsand surface roughness. In this example, the interior surfaces of thecylinders are machined in preparation for the application of a lowfriction, wear resistant coating using plasma transfer wire arc coating,electro-plating, or similar processes.

When used in this specification and claims, the terms “comprises” and“comprising” and variations thereof mean that the specified features,steps or integers are included. The terms are not to be interpreted toexclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately, or in any combination of such features, be utilized forrealizing the invention in diverse forms thereof.

1. A method of casting an article using a mold including a mold cavity,comprising: providing a halogen-containing compound at at least aportion of a surface of the mold adjacent the mold cavity; pouringmolten metal into the mold cavity; cooling and solidifying the metal;and removing the article from the mold.
 2. A method according to claim1, wherein the halogen comprises fluorine.
 3. A method according toclaim 2, wherein the halogen-containing compound comprises dipotassiumfluorotitanate.
 4. A method according to claim 1, wherein the halogencomprises chlorine.
 5. A method according to claim 4, wherein thehalogen-containing compound comprises a potassium chloride-magnesiumchloride eutectic.
 6. A method according to claim 1, wherein thehalogen-containing compound is provided in a coating applied to at leasta portion of a surface of the mold forming at least part of the moldcavity.
 7. A method according to claim 1, wherein the method furthercomprises the step of subjecting the article to isostatic pressure.
 8. Amethod according to claim 7, wherein the method further comprises thestep of maintaining the article at an elevated temperature at which theentire article remains solid while applying the isostatic pressure.
 9. Amethod according to claim 1, wherein the step of providing ahalogen-containing compound at at least a portion of a surface of themold comprises spraying the halogen-containing compound onto the mold.10. A method according to claim 1, wherein the mold is madepredominantly from sand.
 11. A method according to claim 1, wherein thearticle is an engine block.
 12. A method according to claim 11, whereinthe portions of the mold cavity surface that are coated with thehalogen-containing compound form cylinders in the engine block.
 13. Amethod of casting an article using a mold including a mold cavity,comprising: providing a halogen-containing compound at at least aportion of a surface of the mold adjacent the mold cavity; pouringmolten metal into the mold cavity, the metal mostly comprising aluminum;cooling and solidifying the metal; and removing the article from themold.
 14. A method according to claim 13, wherein the metal comprises analuminum-silicon alloy.
 15. A method according to claim 14, wherein themetal comprises a hypoeutectic aluminum-silicon alloy.
 16. A methodaccording to claim 13, wherein the halogen-containing compound comprisesdipotassium fluorotitanate.
 17. A method according to claim 13, whereinthe halogen-containing compound comprises a potassium chloride-magnesiumchloride eutectic.
 18. A method according to claim 13, wherein thehalogen-containing compound is provided in a coating applied to at leasta portion of a surface of the mold forming at least part of the moldcavity.
 19. A method according to claim 13, wherein the method furthercomprises the step of subjecting the article to isostatic pressure. 20.A method according to claim 19, wherein the method further comprises thestep of maintaining the article at an elevated temperature at which theentire article remains solid while applying the isostatic pressure. 21.A cast article formed according to the following steps: providing a molddefining a mold cavity; providing a halogen-containing compound at atleast a portion of a surface of the mold adjacent the mold cavity;pouring molten metal into the mold cavity; cooling and solidifying themetal; and removing the article from the mold.